The present invention relates generally to coating compositions which can be applied to thermoplastic films and, more particularly, to a slip coating composition which, when applied to a thermoplastic film such as polypropylene, results in a film structure having low haze, low coefficient of friction and non-blocking characteristics.
While the present coating composition is suitable for application to a wide variety of thermoplastic films, it is especially suited for application to polypropylene films. Polypropylene films are widely used in the packaging industry due to their physical properties, such as, transparency, stiffness and excellent moisture barrier. With all its good characteristics, unmodified polypropylene film also has the disadvantageous properties of high inherent coefficient of friction and film to film destructive blocking on storage.
In the past, coefficient of friction characteristics of polypropylene and other thermoplastic films have been beneficially modified by the inclusion of fatty acid amides in the polymer. This attempt at minimizing those shortcomings associated with unmodified polypropylene has been described in U.S. Pat. No. 3,176,021. Specifically, it is described that minor quantities of fatty acid amides can be incorporated into the polypropylene in order to improve the coefficient of
In order, however, to obtain the benefits taught by the '021 patent, certain limitations must be observed. The film must be formed from a melt extruded at a temperature between about 400.degree.-550.degree. F. In addition, the amide must be present in from 0.005 to about 2.0 weight percent of the polypropylene, and it must be present along with from 0.1 to about 4.0 weight percent of polyethylene. Under these conditions and limitations, the resulting polypropylene film will have a static coefficient of friction no higher than 0.6 which is significantly higher than present day requirements. In addition, such a film does not have the high stereoregularity required by present day packaging demands. Further, it has been found that once the film has been subjected to temperature conditions approaching 140.degree. F., the coefficient of friction increases significantly and is nowhere near the present day requirements of 0.25.
In addition to the foregoing limitations regarding fatty acid amide-containing polypropylene films, the effectiveness of the amides depends upon their ability to migrate to the surface of the films in order to reduce coefficient of friction. While such amides do improve the coefficient of friction of the films, the value of the coefficient of friction is subject to wide variation depending upon the heat history which the film experiences during shipping, storage, and certain converting processes. The presence of such amides on the film surfaces can adversely affect the film's appearance as manifested by an increase in haze, a decrease in gloss and the presence of streaks. The presence of such amides on the surface can also adversely affect the wettability and adhesion of solvent and water based inks, coatings and adhesives.
In the case of oriented polypropylene films, which are widely used in the food packaging industry, it is common to laminate this film with itself or with other thermoplastic films or with paper films such as glassine paper. When oleamide or erucamide are used in the polypropylene films a significant increase in coefficient of friction has been observed after lamination to such films. It is theorized that this is due either to the migration of the amide back into the polypropylene film or to the loss of the additive layer at the film surface. Therefore, these types of oriented laminated polypropylene films have limited usage for particular converting processes. Attempts at replacing these amide types, in order to provide a consistent coefficient of friction, have not been successful.
Therefore, it is an object of the present invention to provide a coating composition for application to a thermoplastic film, such as polypropylene, whereby the resultant coated film is characterized by a reduced coefficient of friction.
It is another object of the present invention to provide such a coated film, which further exhibits non-blocking and improved slip characteristics.
It is yet another object of the present invention to provide such a coated film having stable non-blocking and improved slip characteristics with respect to the heat history experienced by the film during shipping, storage, converting processes and the like.
It is a further object of the present invention to provide such a film which is additionally characterized by improved clarity resulting from low haze.